The present disclosure relates to an apparatus for driving an inductive load.
Japanese Patent Application Publication No. H02-180007 discloses an apparatus for driving an inductive load. The apparatus for driving the inductive load includes a switching element for driving the inductive load and a current detecting resistor. The apparatus for driving the inductive load converts current flowing through the inductive load via the current detecting resistor to a voltage signal corresponding to the current. Then, to control on/off operation of the switching element, the comparator of the apparatus for driving the inductive load compares the voltage detected by the current detecting resistor with a comparative voltage that is determined so that the voltage detected by the current detecting resistor coincides with the voltage corresponding to a target current. The control circuit of the apparatus for driving the inductive load inputs the voltage detected by the current detecting resistor, which corresponds to the current flowing through the current detecting resistor and digitally controls a duty ratio of the switching element.
Since the switching element drives the inductive load on the high side of the current flowing path of the inductive load, current flowing through the inductive load can be always detected. However, when the switching element drives the inductive load on the low side of the current flowing path of the inductive load and while the switching element is off, current flows through the inductive load by the commutation due to the counter electromotive voltage of the inductive load (reactor). While the switching element is off, current cannot be detected by the current detecting resistor because no current flows through the current detecting resistor. Japanese Patent Application Publication No. 2012-78217 discloses a peak hold circuit so that current flowing through inductive load is continuously detected during off-period of the switching element as well as during on-period of the switching element. The peak hold circuit may be provided between the current detecting resistor and the control circuit, so that the control circuit can detect current flowing through the inductive load based on the output from the peak hold circuit during off-period of the switching element. Specifically, referring to FIG. 5, a switching element 101 and a current detecting resistor 102 are provided in the current flowing path of an inductive load 100. A peak hold circuit 105 is provided between a drive circuit 104 and an amplifier circuit 103 that is connected to the current detecting resistor 102.
As shown in FIG. 5, in the circuit including the peak hold circuit 105 for driving the switching element 101 on the low side of the current flowing path of the inductive load 100, the time constant of the driven inductive load 100 may come closer to the discharge time constant of the peak hold capacitor 105a, with the result that the accuracy of current control can increase. However, the accuracy of current control may deteriorate in the region where the target current value and the duty ratio are small due to the variations of the current detecting resistor 102, the amplifier circuit 103, the peak hold capacitor 105a, and a peak hold resistor 105b. 
The present disclosure is directed to providing an apparatus for driving an inductive load, which can drive the inductive load precisely in a region where the duty ratio is small.